1. Field of the Invention
The present invention relates to a toner for developing electrostatic images, where the toner is well-suited for forming images with electrophotographic methods in a compact energy-efficient high-speed fixing device. The toner provides images with excellent smoothness, glossiness, and high resistance to offsetting. The present invention also related a developer, an image-forming method, and an image-forming device using said toner.
2. Description of the Related Art
Currently, methods for visualizing image data via electrostatic imaging, such as electrophotographic methods, are widely used in various fields. In electrophotography, an electrostatic latent image is formed on the surface of a photoreceptor via a charging step and an exposing step, and the electrostatic latent image is visualized via a developing step.
More specifically, in electrophotography, the latent image on the photoreceptor, which is formed via the charging and exposing steps, is developed with a toner in the developing step, and the resultant toner image is transferred to a recording material such as paper in a transferring step. It is then heated and melted with a fixing member at a high temperature in a fixing step, and is fixed on the surface of the recording material. It is known that, in the fixing step, unless both the toner and the recording material are heated to the minimum necessary temperature or higher by the fixing member, the toner is not fixed on the surface of the recording material.
When the recording material is insufficiently heated, only the portion of the toner which comes into contact with the fixing member is melted and adheres to the fixing member. This phenomenon is referred to as cold-offsetting. In addition, when the recording material is overheated, a hot offset phenomenon is caused, in which the viscosity of the toner is reduced, and a part or all of the toner on the recording material adheres to the fixing member. Therefore, it is necessary for the toner to have a fixing temperature range in which both cold offsetting and hot offsetting are not caused by heating with the fixing member.
In the fixing step, the real surface temperature of the fixing member becomes lower than the set temperature thereof. This is due to heat transfer from the fixing member heated at a high temperature to the recording material, and latent heat necessary for evaporation of a volatile material such as moisture contained in the recording material. Under ordinary circumstances, the real temperature is measured by a temperature sensor and, when the measured value becomes less than a predetermined value, the fixing member is heated again, and returned to the set temperature. However, for example, in the case of continuous fixation, since the amount of heat released from the surface of the fixing member is greater than the amount of heat supplied to the surface of the fixing member, the surface temperature of the fixing member is often lowered.
In recent years, image forming apparatuses are required to have flexibility, which broadens types of paper which can be used in the image forming apparatus. In this case, as described above, the real surface temperature of the fixing member becomes further lower than the set temperature thereof with increase in the weight of a paper sheet. For this reason, the difference between the set temperature and the real surface temperature of the fixing member tends to become greater.
In order to solve this problem, for example, increasing the amount of heat supplied to the surface of the fixing member, and use of a more sensitive temperature sensor disposed in the vicinity of the surface of the fixing member may be adopted. However, these solutions increase cost and cannot generally be adopted. Therefore, the real surface temperature of the fixing member at the time of fixation always varies relative to the set temperature.
On the other hand, in response to the demand for colorization in recent years, a color toner is required to have high developing properties, color mixing properties and high glossiness. It is difficult to separately control these properties and if these properties deteriorate, color reproductivity deteriorates, and the reliability of an image reduces. As described above, the real surface temperature of the fixing member always changes, and the real surface temperature of the fixing member tends to further lower with increase in the weight of a paper sheet and, therefore, there is a tendency that it becomes difficult to stably obtain fixed color images having constant quality, particularly, with regard to glossiness. Therefore, there is a demand for a color toner having wide flexibility for types of paper while maintaining high glossiness.
In order to obtain fixed images having high glossiness, a method of making a molecular weight distribution of a resin contained in the color toner narrow was proposed (see e.g. Japanese Patent Application Laid-Open (JP-A) Nos. 5-158282 and 8-015909). In particular, use of polyester as a toner resin is disclosed (see e.g. JP-A No. 10-097098), since the polyester resin generally can have a sharp melting property, namely a property in which a resin quickly melts when heated, by making the composition of polyester uniform, and obtain high glossiness at a lower fixing temperature.
Since the polyester resin has generally a short side chain of molecules constituting the resin, and the thermal motion of the side chain can be neglected, only the thermal motion of a main chain of the molecules corresponds substantially to the thermal motion of the resin and, therefore, the resin has a sharp melting property. However, when the toner has the sharp melting property and can be melted at a temperature that is too low, preservability of the toner deteriorates, and blocking is caused between toner particles, and such a toner is not preferable. Conversely, when the resin is molten only at a temperature that is too high, fixed images having high glossiness cannot be obtained, and such a toner is not preferable.
Further, control of a chargeability of the toner including the polyester resin tend to be difficult and, for example, the chargeability in summer is often quite different from that in winter. Considering this problem, use of a toner, to the surface of each particles of which fine particles of an inorganic oxide are adhered to reduce change in the chargeability with respect to environment, is disclosed and this method is effective initially. However, the fine particles of the inorganic oxide on the surface of the toner squeeze into the toner, or drop off from the toner surface during use in the form of a developer, and finally the resin properties affect toner characteristics even more. Therefore, it is still difficult to control the chargeability.
In recent years, an emulsification polymerization aggregating method for preparing toner particles having controlled shape and surface composition was proposed (see e.g. JP-A Nos. 63-282752 and 6-250439). In the emulsification polymerization aggregating method, a resin dispersion liquid is prepared by emulsification polymerization, a colorant dispersion liquid in which a colorant is dispersed in a solvent is separately prepared, these dispersion liquids are mixed with each often to form aggregated particles having a diameter corresponding to a toner particle diameter, and the resultant mixture is heated to coalesce the particles to obtain a toner particle. According to this emulsification polymerization aggregating method, it is possible to arbitrarily control a toner shape from an indefinite shape to a spherical shape by selecting heating temperature conditions.
Further, a so-called suspension polymerization method in which a polymerizable monomer, a colorant, a releasing agent and the like are dispersed and suspended in an aqueous medium, and the monomer is polymerized to prepare a toner was proposed. For example, a method for preparing a toner having a multi-layered structure in which a wax as a releasing agent is surrounded by a binder resin is disclosed in JP-A Nos. 8-044111 and 8-286416.
According to these methods, since the particle size distribution of the prepared toner particles can be narrowed, improving image quality is possible, and a toner including a binder resin with a low molecular weight and/or a high molecular weight, which was impossible to prepare by the conventional kneading grinding method, can be prepared.
Therefore, these suspension polymerization methods can easily prepare a toner having a sharp melting property. This is due to the fact that they can reduce the difference between a temperature at which thermal motion of the main chain of the binder resin contained in the toner occurs and a temperature at which thermal motion of side chains of the binder resin occurs. In contrast, in the conventional kneading grinding method, the binder resin contained in the resultant toner must have a molecular weight great enough to enable an additive such as a colorant to be stably dispersed therein. Therefore, thermal motion of the main chain of the binder resin occurs at a higher temperature, and the difference between a temperature at which thermal motion of the main chain occurs and a temperature at which thermal motion of side chains of the binder resin occurs is greater, and it is hard to obtain a toner having a sharp melting property
In addition, a method of reducing uneven glossiness at the time of fixation to obtain a high quality image by defining a change rate of glossiness to temperature was proposed (see e.g., JP-A No. 2000-250258).
Although these methods can afford an excellent toner, it is necessary for toners to contain a component having a less molecular weight to an extent in order to actually impart sharp melting property to a toner. In order to decrease the molecular weight of a binder resin contained in a toner, it is necessary to increase the amount of polymerization initiator at the time of polymerization. In this case, controlling the molecular weight of the binder resin at the time of polymerization tends to be hard and, as a result, it is difficult to obtain the necessary sharp melting property.
In addition, in order to decrease the molecular weight of the binder resin, there exists a method in which a small amount of a surfactant acting as a dispersion stabilizer is added to an aqueous medium. However, when the surfactant is added, not only dose it tend to be difficult to control chargeability, but a washing step also tends to become complicated.
Therefore, there is an increasing demand for a toner having good balance between sharp melting property and chargeability at the time of fixation, and preservability.
Thus, there is a need for a highly glossy toner for developing electrostatic images, in particular full color toner prepared by an emulsification polymerization method or a suspension polymerization method, and a developer in which glossiness at the time of fixation is high, the difference in glossinesses of an image in a recording material (paper) and the difference between glossinesses of images formed on different recording materials are small, and a fixing temperature range at which the resultant toner image can be fixed is wide. In addition, there is a need for an image forming method and an image forming apparatus which can easily and simply form a full color image having high quality and high reliability.